1. Field of the Invention
This invention relates to calibration apparatus, in particular, calibration apparatus for sensor arrays, and more particularly, calibration apparatus for sensor arrays employing optical fiber cable telemetry links.
2. Description of the Prior Art
In applications such as beamforming, it is important to know the phase relationships between the elements in a sensor array. If the array has fiberoptic telemetry links that are distributed over large distances and subject to a variety of stress and thermal conditions, the length of any given fiber can be difficult to predict, thus degrading the phase accuracy of the beamformer. The degradation can be enough to impair adaptive beamforming and nulling.
The amplitude and phase response of each array element will vary with optical losses in the laser-to-detector path due to such factors as, for example, microbending, temperature variations, changes in the filter/gain network response with temperature and aging, stress- and thermally-induced changes in the optical path length, and the like.
Traditionally, there are two ways to calibrate a sensor array. First, a calibration beam may be directed to the sensors in the array. The direction of arrival, frequency, and signal-to-noise ratio of the calibration beam are usually well-characterized, permitting array calibration to within desired tolerances. This method, however, may not be used during normal array operation and, as operating conditions change, calibration data such as fiber length can become stale and inaccurate.
Second, a calibration signal can be transmitted to and redirected from calibration signal detectors in the sensor array. Where the calibration signal, and its return, travel along optical fibers, the phase of the calibration signal can be severely degraded due to thermal effects upon the optical fiber. For example, a typical radar sensor system can have hundreds, or even thousands, of feet of optical fiber cable. A temperature change of, say, 10.degree. F. or 20.degree. F. from some reference temperature can cause the optical fiber cable to vary several inches, or much more, from a corresponding reference length. Such variation can create a substantial phase error in the calibration signal.
In a single pulse, single path calibration system, where the calibration signal is transmitted in only one direction with respect the spatial distribution of the sensors, the actual length of the optical fibers going to each sensor may be unknown due to changing thermal conditions and construction errors. Without this knowledge or a means to compensate therefor, the phase of the calibration signal is essentially uncontrolled. In this situation, a calibration system becomes subject to the same variations as the rest of the sensor system, with attendant adverse effects on system calibration.
Construction errors can arise from situations where optical fiber cables with putatively identical lengths actually differ by several inches, or more, further adding to input signal and calibration signal phase errors. Typically, these construction errors cannot be calibrated out because changing operating conditions can confound calibrations for this factor. As a result, it becomes difficult to compensate for sensor-to-sensor variations in properties of the respective filter/gain networks which can introduce additional phase and amplitude errors.
An array with a self-calibrating system that monitors the receive channels for subsequent digital correction of channel imbalances was described in Hans Steyskal and John F. Rose, Digital Beamforming for Radar Systems, MICROWAVE JOURNAL, January 1989, at 132, 134-36. This system employed a bidirectional loop which provided the calibration source with two signal paths to each elemental receiver for a radio-frequency (RF) calibration signal.
Calibration of an elemental receiver was achieved by two separate measurements of its output signal, with the RF test signal fed through either direction of the loop feed, thereby deriving channel transfer coefficient values that can be independent both of the location of the calibration test port and of the phase shift and attenuation through the feed loop. This system is directed to the use of the calibration system for RF pilot tone distribution in radar applications. In addition, the authors indicate that accurate amplitude and phase references at each element are needed to satisfy the precision requirements of some array systems, and allude to optical fiber technology as the technique of choice for a calibration network.
What is needed, therefore, is a calibration system for a sensor array that can compensate for phase errors such as those imposed by stress- and thermally-induced changes in the lengths of optical fiber connectors between sensors in the array and the sensor signal processor.